Nitroxyl radical is a substance having an unpaired electron, and because of a variety of properties thereof, it is widely used as an antioxidative substance, a chemical cell, a polymerization agent or the like. In addition, nitroxyl radical is highly sensitive to a free radical such as active oxygen, and the distribution of nitroxyl radical in vivo varies depending on the basic structure thereof and the type of substituent. Therefore, utilizing these natures, nitroxyl radical can also be used as a contrast agent for following a free radical reaction in vivo. The present inventors focused their attention on this point and already indicated that novel image analysis can be carried out in vivo by utilizing the simultaneous separate imaging method using 14N-labeled and 15N-labeled compounds (see H. Utsumi, K. Yamada, K. Ichikawa, K. Sakai, Y Kinoshita, S. Matsumoto and M. Nagai, PNAS, 103, 1463 (2006)).
Conventionally, nitroxyl radical is generally synthesized from acetone, ammonia (or ammonia chloride), etc. Typical examples of nitroxyl radicals obtained using this technique include 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) derivatives and 2,2,5,5-tetramethylpyrrolidine-N-oxyl (PROXYL) derivatives.
Reactivities of these conventional nitroxyl radicals to a free radical significantly differ from each other depending on the difference of skeleton (TEMPO-based or PROXYL-based). However, reactivities of TEMPO-based compounds do not differ from each other so much. It is thought that this is because structures around the unpaired electron in nitroxyl radical, i.e., substituents at position-2 and position-6 are not substituted with other substituents.
Several research groups have attempted to carry out replacement of substituents at position-2 and position-6 in a TEMPO-based compound. For example, Miura et al. succeeded in synthesis of 2,6-dispirocyclohexane-4-piperidone using ammonia as a starting material and acetonine as an intermediate (see Y. Miura, N. Nakamura and I. Taniguchi, Macromolecules, 34, 447 (2001)). Wetter et al. succeeded in synthesis of 2,2,6,6-tetraethyl-4-oxo-TEMPO using bisphosphonate as a starting material (see C. Wetter, J. Gierlich, C. A. Knoop, C. Müller, T. Schulte and A. Studer, Chem. Eur. J., 10, 1156 (2004)).
However, these synthesis methods have the following problems: the stability of an intermediate compound such as acetonine is low; many steps for synthesis are required; and there is a lack of application to synthesis of other compounds; etc.